The technique of blown film extrusion is well known for the production of plastic films having a typical thickness between 10 and 300 microns. Plastics, such as low, linear low, and high density polyethylene (LDPE, LLDPE, HDPE) are extruded through a circular die to form a film. Air is introduced through the center of the die to maintain the film in the form of a bubble which increases the diameter of the film about two to six-fold, after which the bubble is collapsed onto rollers.
As the plastic moves away from the die, it begins to cool due to the near ambient temperature of the air blown on the outside surface of the bubble and, in some cases, inside the bubble. The rate of cooling of the bubble is a production rate limiting factor and therefore efforts have been made to increase the rate at which the bubble is cooled.
There are two methods for increasing the cooling rate of existing internal bubble cooling systems. The first is to increase the rate at which air is blown into the bubble. The second method is to lower the temperature of the air fed to the bubble. The first method is limited to airflow rates which do not adversely affect the stability of the bubble. As would be expected, an excessive airflow rate would make it difficult to maintain the shape of the bubble or could result in the collapse of the bubble.
The second method is limited by the physical changes that air undergoes at low temperatures. It has been estimated by thermal analysis, that to obtain a 20% increase in the production rate of blown films, it is necessary to cool the gas entering an internal bubble cooling system to about -100.degree. C., while maintaining the same flow rate. Quite obviously, when air is cooled to such low temperatures the water vapor present in the air will freeze. Existing refrigeration equipment cannot function effectively when such ice formation occurs, and in any case such ice will typically interfere with the surface finish of. It has been found that the air fed to the interior of the bubble can not be cooled to below the dew point of the air (typically about 10.degree. C.) unless extraordinary steps are taken to reduce the amount of water vapor contained in the gas.
Thus, although significant cooling of the gas is desirable to increase production, there has yet to be developed a practical method of introducing extremely cold air into an internal bubble cooling system which is not hindered by the condensation and/or freezing of water vapor.